1. Field of the Invention
This invention relates to a flexible disk jacket for accommodating therein a disk-like magnetic recording medium to form a flexible disk sometimes referred to as a "floppy disk" or "diskette".
2. Description of the Prior Art
As shown in FIG. 1, conventional flexible disks comprise a casing (generally referred to as a "flexible disk jacket") and a disk-like magnetic recording medium 2 accommodated therein for rotation. The flexible disk jacket is formed by folding a plastic sheet 1 such as of polyvinyl chloride (PVC), polycarbonate, polyethylene terephthalate (PET) or the like into a bag-like shape. The recording medium 2 comprises a flexible base sheet which is circular in shape and bears a magnetic recording layer on one side or both sides thereof. To the inner surface of the jacket is attached a liner 3 formed of unwoven fabric, for instance, in order to protect the recording medium 2, reduce rotational torque of the recording medium 2 and clean the surfaces of the same.
When manufacturing the flexible disk jacket, a blank of a predetermined shape is stamped out from a sheet material and then said liner 3 is bonded to the blank by thermolaminating. The resulting laminate is folded, with heat applied to the portions to be bent, and formed into a bag-like shape by heat sealing, ultrasonic sealing or the like.
Therefore, the plastic sheet 1 must have heat sealability. Further, the plastic sheet 1 must be resistant to heat produced when the flexible disk is driven by the recording/reproducing system. The plastic sheet 1 must be dimensionally stable and is preferred to have high scratch resistance.
That is, the plastic sheet 1 must withstand temperatures of up to 80.degree. C. without significant heat deformation or deformation due to heat shrinkage. Further, two parts of the plastic sheet 1 must be heat-sealable together at a temperature between 100.degree. C. and 300.degree. C. with a sufficient bonding strength, and the liner 3 of such as unwoven fabric of rayon/propylene resin or polyester resin must be heat-sealable to the plastic sheet 1 at a temperature in the same range with a sufficient bonding strength.
Conventional flexible disk jackets are generally colored substantially in achromatic color, e.g., black or other dark color. This is because carbon is added to the plastic sheet 1 as an antistatic agent. When the jacket is electrostatically charged, dust is attracted thereto to cause drop-out upon recording or reproduction or to cause the surface of the recording medium 2 to be scratched.
Carbon is advantageous over other antistatic agents in that it can, in small amounts, reduce the electrical resistance of polyvinyl chloride without adversely affecting the properties of the polyvinyl chloride, an inexpensive and easy-to-process material that is widely used for the plastic sheet 1 in producing the jacket. Furthermore carbon itself is inexpensive and stable and does not adversely affect the magnetic recording medium 2.
However, there is a demand for flexible disk jackets colored in various fresh chromatic colors as such flexible disks would be more attractive in appearance and could be easily separated by color. Further, flexible disk jackets of dark color are disadvantageous in that any fingerprints are clearly visible. In order to color the flexible disk jacket in a fresh chromatic color, a colorless or thin white antistatic agent must be used. However, such antistatic agents are apt to exude from the surface of the jacket to be transferred to the surface of the recording medium, thereby adversely affecting the properties and durability of the recording medium, and at the same time the electric resistance of such antistatic agents is apt to change with time depending on the storage conditions.
Further, sometimes, the flexible disk jacket must have light-shielding properties.
In short, there has been developed no plastic sheet material which can satisfy all the requirements for the flexible disk jacket.